1. Field of the Invention
The present invention relates to the inhibition of scale formation from sodium alkali containing brines. More specifically, the invention relates to the use of alkylbenzene sulfonic acids, or salts thereof, to inhibit scale formation on processing and handling equipment contacting sodium alkali containing brines.
2. State of the Art
Soda ash and other sodium containing products may be produced from Trona (Na2CO3.NaHCO3.2H2O), or other sodium containing ores (e.g. nahcolite, NaHCO3), using well known dry-mining or solution-mining processes. In either case, the sodium containing ore is dissolved in an aqueous media to form a sodium alkali containing brine. Representative examples of such dry-mining and solution-mining processes are described in U.S. Pat. No. 4,557,910 issued to Meadow, U.S. Pat. No. 5,262,134 issued to Frint et al., U.S. Pat. No. 5,283,054 issued to Copenhafer et al., and U.S. Pat. No. 5,766,270 issued to Neuman et al.
An unfortunate and highly disadvantageous problem of both the dry and solution mined processes is the formation of scale on the surfaces of equipment used to handle and process the sodium alkali containing brines formed in such processes. For example, magnesium containing minerals are often present in sodium carbonate and/or bicarbonate containing ores as impurities. Some of these are soluble in aqueous media. As a sodium containing ore is dissolved to form a sodium alkali containing brine, some of the magnesium and other polyvalent metals containing minerals are also dissolved in the brine. When the equilibrium solubility of magnesium or such other metals in the brine is exceeded, scale formation is likely to result. This may occur with the elevation of the brine temperature or elevation of the magnesium concentration in the brine. Because magnesium solubility decreases with temperature, magnesium scale formation is particularly exacerbated by the elevated temperatures encountered in soda ash production processes. However, scale may also form at ambient temperatures, although at a slower rate. The formation of magnesium scale in the process impedes production and hinders process efficiency.
Scale formation in soda ash production processes includes the formation of both crystalline salt scales and amorphous scales. Oftentimes, the scales contain magnesium, calcium, silicon, and/or other divalent metals, especially alkaline earth metals. Some common magnesium containing scales include eitelite (Na2CO3.MgCO3) and bradleyite (Na3Mg(PO4)(CO3)). Calcium containing scales, such as pirssonite (Na2CO3.CaCO3.2H2O), and silicon containing scales, such as analcime (NaAlSi2O6.H2O), are also found in soda ash production processes. Scales may form from compositions of these various chemical compounds.
The scale formations generally found in equipment handling sodium carbonate and/or bicarbonate brines have essentially contained polyvalent metal cations and carbonate or oxide anions as well as sodium cations. Sulfate containing scales have generally not been a problem in handling sodium alkali brines used in the ultimate production of soda ash or other sodium containing products.
Typically, scale forms on the surfaces of heat exchangers, pipes, filters, tanks, clarifiers, valves, or any other equipment in contact with sodium alkali containing brines or slurries in soda ash production processes. Scale must be removed from the process equipment on a regular basis to maintain acceptable process conditions and efficiencies. Scale removal is usually accomplished by acid cleaning. This requires a shutdown of the process while the equipment is dismantled, cleaned, and put back together. Such shutdowns are both expensive and time consuming.
In an attempt to decrease the amount of cleaning required, and thus the number of process shutdowns, scale inhibitors are added to the brine streams. A number of different scale inhibitors are currently used to reduce the amount and rate of scale formation on process equipment. Some examples of scale inhibiting additives include HEDPA (1-hydroxyethylidene-1,1-diphosphonic acid), ATMP (aminotri(methylenephosphonic acid)), DETPMP (diethylenetriaminepentakis(methylenephosphonic acid)), and polycarboxylate and polyphosphonate polymers and copolymers. Such additives, however, are often costly and may reside as contaminates in the sodium containing products.
Other chemicals may be added to the sodium alkali containing brines in soda ash production processes as well. Although it has not been used as a scale inhibitor in sodium alkali brine processes, dodecylbenzene sulfonic acid has been added to sodium sesquicarbonate crystallizers to improve the crystal habit of sodium sesquicarbonate crystals formed within the crystallizer. Addition of the dodecylbenzene sulfonic acid commonly occurs within the crystallizer. However, dodecylbenzene sulfonic acid has also been added to a crystallizer-ready sodium alkali containing brine at the point where the crystallizer-ready sodium alkali containing brine is fed to a crystallizer. Dodecylbenzene sulfonic acid has heretofore not been introduced anywhere else in the brine handling portion of a soda ash production process.
In an unrelated field, see U.S. Pat. No. 5,156,706, dodecylbenzene sulfonic acid has been disclosed as a scale inhibitor for sulfate type scales formed in waste water evaporators wherein the scale was identified as a calcium sulfate containing scale.
A cost-effective, benign scale inhibitor for use in sodium alkali containing brines which inhibits the formation of scales associated with the production of soda ash from such brines without adversly affecting other aspects of such brine processes is desirable to decrease the frequency, cost and time associated with process shutdowns for scale removal.
In one aspect of the invention, alkylbenzene sulfonic acids, or salts thereof, are added to sodium alkali containing brines to inhibit scale formation from the brines caused by polyvalent metal ions and carbonate and/or silicate and/or oxide anions in the presence of sodium cations, onto processing and handling equipment in contact with the brines. In the specification and the claims, the term xe2x80x9csodium alkali containing brinexe2x80x9d includes brines comprising sodium carbonate, sodium bicarbonate and/or a combination of sodium carbonate and sodium bicarbonate. Such brines may be processed to form crystals of sodium carbonate, sodium bicarbonate, and/or crystals of carbonate and bicarbonate in physical combination or in chemical combination such as sodium sesquicarbonate. The crystals may be hydrated or non-hydrated.
In one embodiment of the present invention, dodecylbenzene sulfonic acid, or its salt (individually and collectively xe2x80x9cDDBSAxe2x80x9d), is added to a sodium alkali containing brine in a sodium sesquicarbonate production process to prevent the formation of sodium containing, polyvalent metal containing salts, such as eitelite scale on production and handling equipment. Although DDBSA may be added to a crystallizer, or to a crystallizer-ready sodium alkali containing brine, to improve crystallization habit in a sodium sesquicarbonate production process, DDBSA has heretofore not been added upstream of the crystallizer and heretofore has not been added for the intentional purpose of scale inhibition.
In the present invention, DDBSA is added upstream of the crystallizer to inhibit scale formation in piping and equipment leading to the crystalizer. The addition of DDBSA to a sodium alkali containing brine decreases the formation of eitelite or other complex carbonate scale on pipes, filters, tanks, and other production equipment which come in contact with sodium alkali containing brines used in various sodium carbonate and/or sodium bicarbonate production processes. It is theorized that the DDBSA alters the crystalline structure of eitelite scale, if and when it forms, resulting in a less adherent crystal which is easily removed from process equipment. The addition of DDBSA to sodium alkali brine streams also hinders or prevents the formation of calcium containing scales, magnesium containing scales, carbonate containing scales and silicon containing scales (e.g. pirssonite, bradleyite, and analcime).
Alkylbenzene sulfonic acids, such as DDBSA, are also useful as scale inhibitors in other processes involving sodium alkali containing brine solutions and slurries. For example, alkylbenzene sulfonic acids may be used to inhibit scale formations in pipes used to transport or pump sodium alkali containing brines from one location to another location. Alkylbenzene sulfonic acids are useful as scale inhibitors in sodium carbonate monohydrate, sodium carbonate decahydrate, sodium bicarbonate and sodium sesquicarbonate production processes or other sodium containing crystal production wherein process equipment and transporting piping is exposed to sodium alkali containing brines.
The term xe2x80x9cinhibitorxe2x80x9d or xe2x80x9cinhibitionxe2x80x9d as used herein means that scale formation quantity is reduced or the character of the scale formation is changed, e.g. the adherent nature of the scale is reduced or the scale is less dense, any or all of which effects render the scale easier to remove and/or be more quickly removed.
The alkylbenzence sulfonic acids may be added to the sodium alkali brines in salt form, e.g. as sodium or other alkali metal salts, ammonium salts, or similar water soluble salts. It will be recognized, also, that such salts may inherently form when the alkylbenzene sulfonic acids are added to the brines.